Do you think there is any way for the dam itself at Oroville to fail? Nobody says that there is a chance of that publically, and looking at the way the spillways are separated from the dam by the hillside, it doesn’t look like it could happen. I don’t think it could happen from erosion coming from the spillways. I do think that there will be tremendous erosion from the spillways which will continue, especially if the water continues around the emergency spillway and goes over the parking lot.
The magnitude of such a breach depends entirely on how far down any erodable rock veins go below the emergency spillway. The latter is 30 feet high so the least magnitude of a breach would be from its top edge (901' above sea level) to 871' above sea level. The maximum credible estimate I've seen is 100' down, which would be to 801' above sea level.
The chief moderator of the professional forum I've followed estimated the volume of reservoir water from 801' feet elevation to 901' is 1.2 million acre feet of water. That is the probable worst case. My guesstimate of a 30' breach from that is 400,000 - 500,000 acre feet.
Either volume wouldn't all go at once. IMO it would take hours for a breach to form from the time the emergency spillway overflows, more hours for the breach to erode down as far it can go, and still more hours for the reservoir to drain as much as it's going to.